Carcinoma of the prostate (CaP) is the second leading cause of cancer related death in men. CaP progresses from an androgen dependent (AD) stage to an androgen- independent (AI) stage after androgen ablation therapy. Androgen receptor (AR), a hormone regulated transcription factor, is crucial for the progression and maintenance of both stages and thus is a validated therapeutic target to treat this disease. An AR antagonist, bicalutamide, is currently utilized quite successfully in the clinic and several other new AR antagonists are currently in preclinical and clinical development. In addition to AR, growth factor driven epidermal growth factor receptor (EGFR) signaling pathway, also plays a role in the progression of AI prostate cancer by supporting AR transcriptional activity. For this reason, alternative strategies to treat prostate cancer have targeted receptor members of the EGFR pathway but have met with limited clinical success as single agents. Our broad, long term objective is to dissect the functional interaction between AR and the EGFR signaling pathway as it relates to progression of AI prostate cancer. We have discovered a paradoxical but novel role of an EGFR pathway-specific growth factor ligand, heregulin, in inhibiting AR transcriptional activity in a prostate cancer system. Heregulin-mediated AR inhibitory activity is dependent upon activation of the receptor partners of heregulin. Consistent with these data, we propose to develop a cell based highthroughput assay in a prostate cancer system to screen for novel small molecule or peptide entities that function as AR antagonists utilizing the EGFR pathway. This will be achieved by stably integrating the androgen receptor DNA response element-driven luminescent reporter gene within the genome of a prostate cancer cell line. Such a system will be utilized to optimize AR transactivation assays and subsequent heregulin- mediated inhibition of AR transactivation. Counter-screening assays and secondary assays will help understand the mode of action of the identified compounds and suggest ways to triage them. AR antagonists identified using the AR transcription-based assay will have a potential clinical value in AI prostate cancer, in addition to serving as research tools to delineate the fine intricacies between the EGFR and AR signaling pathways. Project Narrative: Therapies that attack proteins important for prostate cancer progression hold great promise for the successful treatment of this disease. Androgen receptor is one such protein. Our research proposes to develop methods to search for compounds that specifically block the androgen receptor in prostate cancer cells. [unreadable]